Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
420 AM EDT Fri Apr 16 2021

Valid 12Z Fri Apr 16 2021 - 12Z Mon Apr 19 2021

...Northeast...
Days 1...

A late season Nor'easter will bring heavy snow to the Green
Mountains of VT and White Mountains of NH, and adjacent CT River
Valley along the NH/VT border.
Ascent is favored in the proximity of an upper level jet with 300
mb divergence/700 mb convergence bands crossing NH and VT. Mixed
precipitation is expected elsewhere in southeast NH and downeast
Maine. Initial temperatures are well above freezing in southern
Maine so much of the precip early today falls as rain before
cooling later leads to a brief period of snow.  Snow this morning
in the Worcester Hills and Berkshires of MA should taper as drier
air aloft arrives this afternoon.
Snow is expected to continue on the east facing slopes of the NY
Adirondacks with several inches possible. The models now cluster
better with the 00z NAM having shifted north with its low track
and precip shield vs last night's forecast.  The consensus has the
low reaching southeast MA or Cape Cod through tonight and then
moving off the coast Saturday, with the event ending as a result.

...CO/NM Rockies and UT Wasatch...
Days 1-3...

An upper trough persists across northern CO and UT today, drifting
south. Upper divergence/700 mb convergence maxima develop over the
UT Wasatch/Uintas today and downstream crossing the San Juans of
southwest CO and northern NM tonight and tomorrow, reaching the
Sangre De Cristo Mountains by Saturday night. Deep layer moisture
remains in place, so even though the sfc-600 mb winds are light,
the lift in the moist airmass is sufficient to produce periods of
snow in these mountain ranges, with several inches expected.
The long duration of favorable 700 mb convergence across the CO
San Juans leads to 2 day totals of 1-2 feet across favored terrain.
The 700 mb front/corresponding upper trough moves south on Day 3
and a respite develops as the moisture pool with the front moves
south across New Mexico.
Snow probabilities for 4 or more inches for both day 1 and day 2
are moderate to high for the San Juans and Sangre De Christos of
CO/NM.

...Montana/Wyoming...
Day 3...

A 700 mb shortwave trough and associated sfc cold front drops
south across Montana on Sunday, reaching Wyoming by Monday
morning. A band of enhanced sfc-700 mb moisture convergence and
ascent with the front leads to a period of snow, with higher
amounts limited to upslope areas of the MT Rockies, including
Glacier National Park and the Beartooth Mountains. As the front
underneath the 300 mb jet maxima continues into Wyoming, enhanced
amounts are expected in windward terrain in the Absarokas and the
Bighorns Mountains, where as much as a foot is possible.  Day 3
snow probabilities are high for 4 inches and low for 8 inches in
most areas, except moderate in the higher terrain of the Big Horns
of WY.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen